In the present era of world trade and managed distribution, consumers enjoy a wide assortment of fresh produce at their local grocery stores throughout the year. The fresh produce includes fruits and vegetables that are in season and out of season, harvested locally and grown overseas. At a given time of the year, a store in Omaha, Nebr. might have in stock apples from New Zealand, bananas from Costa Rica, pineapples from the Maui, lettuce from California, and corn from neighboring Nebraska counties. The convenience of year round produce is accomplished by sophisticated planning and distribution networks which take unripened produce directly from harvest, timely ripen it, and then transport the produce to stores for sale at the height of its ripeness and freshness.
It is well known that certain perishable produce can be maintained in fresher condition at refrigerated temperatures above freezing. Controlled atmospheres which inhibit rapid ripening and spoilage are also used to enhance freshness. It is therefore common to store and ship produce in refrigerated and/or controlled atmosphere structures to maintain the freshness, particularly in view of the large distances and long time spans often required for shipping, importation, and distribution.
Common controlled atmospheres are created within shipping or storage facilities by periodically flushing the enclosed environment with an inert gas, such as nitrogen. Due to the presence of the predominate inert gas, the controlled atmosphere has a small quantity of oxygen, thereby slowing the ripening process of the produce.
Consider the trek of bananas, a representative imported fresh produce, from harvest to point of purchase. The bananas are harvested from trees in tropical habitats (such as Costa Rica) in an unripened condition. The bananas are placed on a large cargo ship and transported to a port in the United States, such as Los Angeles or Tampa Bay. The cargo ship has special chambers with controlled atmospheres designed to prevent the bananas from ripening. Once in the U.S., the bananas are stored temporarily in large controlled atmosphere warehouses during importation procedures. The bananas are then transferred to a truck trailer that is also specially designed to maintain a controlled atmosphere which inhibits ripening of the bananas. The bananas are ported in the truck trailer to another warehouse facility for ripening.
The ripening facility provides an environment for controllably ripening the bananas. The environment is primarily comprised of nitrogen, with small parts of oxygen. To ripen the bananas, ethylene C.sub.2 H.sub.4 is introduced into the controlled atmosphere. During ripening, the bananas absorb oxygen and simultaneously release carbon dioxide CO.sub.2, a process known as "respiration". When the bananas are just about ripe, they are loaded back onto refrigerated truck trailers and distributed to the grocery stores for sale. The bananas are ripe, or slightly unripe, by the time they are displayed in the grocery stores.
Example refrigerated and/or controlled atmosphere systems and containers are described in the following patents. U.S. Pat. No. 4,716,739 describes a system that initially flushes its container with a nitrogen gas to reduce the oxygen level to a base amount, and thereafter controls oxygen levels within the container by selected inflow of ambient air. U.S. Pat. No. 5,333,394 discloses a controlled atmosphere system that manages atmospheric conditions within multiple, gas tight, interchangeable container modules. U.S. Pat. No. 4,961,332 discloses a food storing device having a cooling mechanism, a supply of nitrogen for controlling levels of oxygen within the storage device, and a lamp to facilitate photosynthesis.
Prior art portable controlled atmosphere systems typically have a limitation in that they maintain the controlled atmosphere according to a chosen setting. For instance, U.S. Pat. No. 3,239,360 describes periodically purging a container with nitrogen gas in response to oxygen levels within the container to maintain a specified oxygen content. The controller in the '360 patent maintains the oxygen levels within the container at a fixed set point between 1/2% and 4% by volume. Unfortunately, fresh produce changes during transportation due to numerous parameters such as varying respiration rates, temperature, and transportation time. As a result, the prescribed oxygen levels suitable for maintaining the desired environment might change at various stages of the trip.
Furthermore, there has yet to be developed a workable portable container that controllably ripens fruit during transportation. The ripening process causes certain fruits and vegetables to release large amounts of heat at a rapid pace. The refrigeration system on prior art portable containers are incapable of managing the heat and the produce prematurely spoils in route. Thus, the fruit is typically ripened in large warehouse ripening facilities which have large refrigeration systems to accommodate the heat released during ripening. Thereafter, the fruit is loaded back onto a refrigerated, controlled atmosphere truck for regional distribution.
It would be desirable to provide a portable controlled atmosphere system that adapts to the evolving climate within the controlled produce environment. It would further be beneficial if the portable controlled atmosphere system could be configured to inhibit ripening, promote controlled ripening during transportation, or a combination of both. Such a system would decrease distribution costs by reducing or eliminating use of warehouse ripening facilities.